Intermolecular Forces: Strength, Types, and Examples
Intermolecular Forces Notes
Intermolecular Forces
o
Intermolecular Forces
: attraction
between
molecules
o
Much
weaker
than
chemical
bonds
Strength of Forces
•
STRONGEST
•
Covalent Bonds (400 kcal)
•
Hydrogen Bonding (12-16 kcal)
•
Dipole-Dipole Interactions (0.5-2 kcal)
•
London Dispersion Forces (<1 kcal)
•
WEAKEST
•
kcal = kilocalorie (unit of energy stored in chemical bonds)
London/Van der Waals
o
London Dispersion
Forces:
o
Attraction due to the
constant motion of
electrons
o
Will cause
temporary
concentration of charge
on one side of an
atom/molecule
o
Exist
between ALL
molecules
Dipole
o
Dipole-dipole:
o
Attraction between
polar
molecules
The squiggly symbol
means a partial charge.
An Analogy
•
Dipole-dipole attraction
•
- - - - - - -
•
The two couples are very happy with each
other, but the guy is attracted to the other girl.
Hydrogen
o
Hydrogen Bonding:
o
Stronger type of dipole-
dipole
interaction
o
Occurs only between
molecules with
lone pairs
on center atom AND
hydrogen bonded to F, O,
or N
o
Hydrogen bonding is
FON
!
Is the molecule polar?
No
Yes
London Dispersion Forces
Does the molecule have
lone pairs on the center
atom, with H bonded to F,
O, or N?
Yes
No
London Dispersion Forces
Dipole-dipole
Hydrogen bonding
London Dispersion Forces
Dipole-dipole
Intermolecular Forces
Intermolecular Forces
Examples
Cl
2
•
The molecule is
nonpolar.
•
This molecule has
London Dispersion Forces
.
Intermolecular Forces
Examples
•
CH
3
I
•
The molecule is
polar
.
•
The central atom does not have lone pairs, and H is not
bonded to F, O, or N.
•
The molecule has
London and Dipole-dipole forces
.
Intermolecular Forces
Examples
•
CH
2
O
•
The molecule is
polar
.
•
The central atom does not have lone pairs, and H is not
bonded to F, O, or N.
•
The molecule has
London
and
Dipole-dipole forces
.
Intermolecular Forces
Examples
•
O
3
•
The molecule is
polar.
•
The central atom has a lone pair, but does not
have H bonded to F, O, or N.
•
The molecule has
London
and
Dipole-dipole
forces
.
Intermolecular Forces
Examples
•
PF
3
•
The molecule is
polar.
•
The central atom has a lone pair, but does not have H
bonded to F, O, or N.
•
The molecule has
London
and
Dipole-dipole forces
.
Intermolecular Forces
Examples
•
NH
4
+
•
The molecule is
nonpolar…
•
So even though H is bonded to N…
•
This molecule has only
London Dispersion forces.
Intermolecular Forces
Examples EXTRA
•
H
2
O
•
The molecule is
polar.
•
There is a lone pair.
•
Hydrogen is bonded to F, O, or N.
•
This molecule has
London, Dipole-Dipole forces
AND hydrogen bonding.
Intermolecular forces are attractions between molecules, weaker than chemical bonds. They include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Strength varies, with covalent bonds being the strongest and London dispersion forces the weakest. Different types of intermolecular forces depend on molecule polarity and presence of lone pairs. Examples like Cl2 and CH3I illustrate these concepts in action.
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Presentation Transcript
Intermolecular Forces o Intermolecular Forces: attraction between molecules o Much weaker than chemical bonds
Strength of Forces STRONGEST Covalent Bonds (400 kcal) Hydrogen Bonding (12-16 kcal) Dipole-Dipole Interactions (0.5-2 kcal) London Dispersion Forces (<1 kcal) WEAKEST kcal = kilocalorie (unit of energy stored in chemical bonds)
London/Van der Waals o London Dispersion Forces: o Attraction due to the constant motion of electrons oWill cause temporary concentration of charge on one side of an atom/molecule o Exist between ALL molecules
Dipole o Dipole-dipole: o Attraction between polar molecules The squiggly symbol means a partial charge.
An Analogy Dipole-dipole attraction - - - - - - - The two couples are very happy with each other, but the guy is attracted to the other girl.
Hydrogen o Hydrogen Bonding: o Stronger type of dipole- dipole interaction o Occurs only between molecules with lone pairs on center atom AND hydrogen bonded to F, O, or N oHydrogen bonding is FON!
Intermolecular Forces Is the molecule polar? No Yes Does the molecule have lone pairs on the center atom, with H bonded to F, O, or N? London Dispersion Forces No Yes London Dispersion Forces Dipole-dipole London Dispersion Forces Dipole-dipole Hydrogen bonding
Intermolecular ForcesExamples Cl2 The molecule is nonpolar. This molecule has London Dispersion Forces.
Intermolecular ForcesExamples CH3I The molecule is polar. The central atom does not have lone pairs, and H is not bonded to F, O, or N. The molecule has London and Dipole-dipole forces.
Intermolecular ForcesExamples CH2O The molecule is polar. The central atom does not have lone pairs, and H is not bonded to F, O, or N. The molecule has London and Dipole-dipole forces.
Intermolecular ForcesExamples O3 The molecule is polar. The central atom has a lone pair, but does not have H bonded to F, O, or N. The molecule has London and Dipole-dipole forces.
Intermolecular ForcesExamples PF3 The molecule is polar. The central atom has a lone pair, but does not have H bonded to F, O, or N. The molecule has London and Dipole-dipole forces.
Intermolecular ForcesExamples NH4+ The molecule is nonpolar So even though H is bonded to N This molecule has only London Dispersion forces.
Intermolecular ForcesExamples EXTRA H2O The molecule is polar. There is a lone pair. Hydrogen is bonded to F, O, or N. This molecule has London, Dipole-Dipole forces AND hydrogen bonding.
